In most, if not all, processes involving chemical conversions the control of temperature by means for transfer of energy is very important, because chemical reactions either absorb or evolve energy. Where highly exothermic reactions are carried out in a flow reactor containing a fixed heterogeneous catalyst, energy evolved near the entrance of the reactants in contact with the catalyst is well-known to cause non-isothermal conditions which can result in deleterious overheating of the catalyst. Furthermore, non-isothermal conditions of reaction are likely to decrease desired conversions, throughput, and/or yields of value added products.
In a large class of industrial processes the conventional design of reaction apparatus applicable for use in carrying out highly exothermic chemical reactions uses an annular bundle of vertical contact tubes which are adapted to contain a fixed heterogeneous catalyst. Reaction gases are directed through the tubes containing the catalyst and the heat evolved as the reaction proceeds is removed by a heat carrier which is circulated over the outer surface of the contact tubes.
Conventional flow reactors are illustrated in FIG. 1. Typically, such reactors comprise a plurality of walled conduits each having an outer surface disposed for contact with a heat-transfer medium, and an inlet distribution manifold adapted for flow communication with a downstream manifold. The conduits are of uniform cross-section throughout the length of the reactor. In such reactors, it is sometimes difficult to balance the heat generated during the reaction with the heat removal capabilities of the heat transfer medium. The result is that such reactors may operate with a “hot spot” (i.e. the location in the catalyst bed wherein the exothermic reaction exceeds the heat removal capabilities of the reactor), or “runaway reactions” (because of insufficient heat removal and often wherein oxygen is a reactant, the reactants and preferred products continue to oxidize or combust to non product chemical compounds). Either of these occurrences often leads to an irreversible chemical or physical damage of the catalyst and/or drastically reduces the life and/or performance of the catalyst. Specifically, the catalyst may melt and/or fuse together; the catalyst crystal structure or composition may be altered, any of which can cause the loss of activity and/or selectivity of the catalyst to preferred products.
Many designs directed to an improved heat exchange arrangement for such reaction apparatus are known.
For example, U.S. Pat. No. 3,850,233 in the name of Oskar Wanka and Jeno Mihaleyi describes reaction apparatus which is a compact structure of a closed type, without external portions, but with a complex internal arrangement including a pump which directs a flow of heat carrier medium along an inner tubular baffle toward the opposite end and then through an opening in the inner tubular baffle over the contact tubes and then back toward the pump for return through an annular space between an annular baffle and the inner tubular baffle. This complex internal arrangement is said to provide a most favorable flowing course for the heat carrier and a desirable heat exchange relationship between the different media for endothermic chemical processes.
U.S. Pat. No. 3,871,445 in the name of Oskar Wanka, Friedrich Gutlhuber and Hermann Graf describes conventional design of reaction apparatus for carrying out exothermic and endothermic chemical reactions, having a shell in which there is arranged a vertical nest of contact tubes. These contact tubes, which contain a catalyst material, have their opposite ends secured, in a fluid-tight manner, into respective headers and open, at their opposite ends, into upper and lower heads connected to the shell, reaction gases flowing through the contact tubes are supplied and removed through these heads. According to the patent, a heat exchange medium is pumped through an external heat exchanger and is supplied and discharged to the shell through respective axially spaced annular supply and discharge conduits, to flow over the contact tubes. Baffles are arranged in the shell to extend transversely to the length of the tubes to direct the heat exchange medium to flow alternately in opposed radial directions over the tubes between the supply and discharge conduits. At least one additional annular circuit is arranged at a point of the shell intermediate between the supply and discharge conduits, is connected to the heat exchanger and the shell, and supplies and discharges a partial amount of the heat exchange medium. In one of these complex examples, several such additional annular conduits are arranged at respective points of the shell intermediate between the supply and discharge conduits. In another, diaphragms or partitions divide the shell side into separate compartments each of which has a respective heat exchanger associated therewith.
More recently, U.S. Pat. No. 3,898,295 in the name of Oskar Wanka, Friedrich Gutlhuber and Cedomil Persic describes a multistage reaction apparatus for carrying out exothermic or endothermic catalyst reactions comprising a plurality of separate stages which are arranged sequentially within the reaction vessel and consecutively passed through by the reaction gas. Each stage includes a separately removable module filled with a catalyst, and a gas cooler in the form of a heat exchanger mounted downstream of the module. Each heat exchanger represents a controllable partial cooling circuit and all of the exchangers are interconnected by a common circulation system serving to balance out larger temperature variations and to supply the partial circuits. The common circuit, including a main heat exchanger and a pump mounted in the return branch or branches of the circuit and the partial circuits or exchangers are controlled by valves or three-way control members and may also each comprise a pump. According to the patent such complex multistage reaction apparatus for carrying out exothermic or endothermic catalytic reactions in which the reaction gas subsequently passes through several beds of catalysts placed in transversely arranged cases and is cooled down or heated up in each such stage by means of a heat exchanger whose partial medium circuit is controllable by valves or three-way control members and with the aid of a main circulation system is thereby capable to hold the temperature of the reaction gas uniformly distributed over the cross-section of the reactor and, at the entrance of the stages, on the substantially same level.
U.S. Pat. No. 4,657,741 in the name of Rudolf Vogl, describes a reactor for carrying out exothermic and endothermic catalytic reactions which includes a contact tube bundle and radial admission and removal of a heat transfer medium via an annular duct for each, and a circulation through an external heat exchanger. Two or more circulating pumps are connected to the annular ducts and are distributed over the circumference. The heat exchanger can be arranged in shunt to the main circulation and be connected with individual sections of at least one annular duct via setting elements.
In U.S. Pat. No. 5,161,605 in the name of Friedrich Gutlhuber a tubular reactor for catalytic gas-phase reactions is described with symparallel (sic) guidance of the heat exchanger. A partial stream of the heat-exchanger medium, immediately neighboring the inlet side of the tube plate, is introduced through a by-pass channel arranged in the center of the bank of tubes, by-passing the bank of tubes, and at a point downstream of the discharge area of the heat-exchanger. In this way, according to the patent, undesirable severe local cooling in the reaction area of the, bank of tubes can be avoided.
All the above-described methods are essentially based on modifying the heat transfer from the contact tubes which contain heterogeneous catalyst after this heat has been produced by the chemical conversion reactions therein. In a paper titled, “An Alternative Method to Control the Longitudinal Temperature Profile in Packed Tubular Reactions (ING. CHIM. ITAL., v. 12, n.1-2, pp. 516, gennaio-febbraio 1976) authors P. Fontana and B. Canepa credit P. H. Chalderbank, A. Caldwell and G. Ross as suggesting another method whereby the heat generation rate is controlled at the source, by mixing catalyst-containing pellets and inert pellets in variable ratio along the axial co-ordinate. See “Proceedings of the 45 h European Symposium on Chemical Reaction Engineering” (Pergamon Press, London 1971). Charging a plurality of contact tubes with a mixture of catalyst-containing pellets and inert pellets according to a prescribed variable ratio along the axial co-ordinate, clearly complicates the loading process as well as recovery of catalyst values from deactivated catalyst. Whether or not such a method could in any way be more useful than previous described methods, it is clearly based on the regulation of the heat produced per unit of time and volume of the bed without altering the means for transfer of such heat from the outer surface of the tubes.
Authors Fontana and Canepa direct their paper to a method of obtaining a predetermined axial temperature profile by replacing the inert pellets of Chalderbank et al, with a coaxial inert body which makes the cross section, taken up by the active catalyst pellets, annular and variable along the axial co-ordinate. In a theoretical example, based upon their reduction of a mathematical model into a one-dimensional form, for an irreversible exothermic reaction of A+B going to C with B in large excess, a complex longitudinal profile of an axial inert body is shown as a graph. According to their mathematical analysis, the complex longitudinal profile derived for the axial inert body should, in theory at least, realize a constant longitudinal temperature profile. Where a typical commercial reactor for a highly exothermic conversion contains up to 20,000 or even 30,000 contact tubes which are long relative, e.g., 100 to 250 times their diameter, there remain unsolved mechanical problems involving fabrication and/or maintenance of a coaxial inert body in each tube, as well as in loading catalyst into an annular space from the end with smallest dimension.
Other methods of obtaining a predetermined temperature profile along the axial co-ordinate of flow reactor containing a fixed heterogeneous catalyst is a quench-type reactor wherein cold fluid, such as fresh an/or recycled reactant, is injected into the flow at a plurality of points along the axial co-ordinate or between a plurality of catalyst beds. However, in a paper titled “Technology of Lurgi's Low Pressure Methanol Process” (CHEMTECH, July, 973, pp. 430-435) author E. Supp demonstrates that for methanol production from carbon oxides and hydrogen the tubular reactor with boiling water around the tubes provides more constant temperatures than does a quench-type reactor. Moreover, the temperature profile on the tubular reactor drops toward the outlet and thus contributes to a better equilibrium, while each stage of the quench-type reactor has an increasing temperature profile.
German Patent No. 29 29 300 describes a catalytic reactor, for use in carrying out endothermic or exothermic reactions, through which a reactant fluid is flowed, and containing a reaction chamber filled with catalyst material, which is in thermal contact with a heat-emitting or heat-absorbing fluid, and characterized by the fact that the cross-section surface area of the reaction chamber is varied, along with the direction of flow of the reacting fluid, depending upon the quantity of heat required for completion of a given reaction, or the quantity of heat released on the course of a reaction. For a proposed methanol synthesis reactor, the diameter of the reaction chamber is varied along the direction of flow of the reacting fluid such that the diameter (in mm), is a constant, having a value of from 15 to 25, multiplied by the gas flow rate per reaction tube (Nm3/hr) raised to the power of a constant having a value of from 0.12 to 0.22. As practical matter, the reaction chamber is made up of only from 2 to 5 sections of tubing having constant diameter.
Japanese Patent No. 61-54229 describes a chemical reactor for exothermic conversions to form methanol which reactor has of a vertical reaction column filled with a granulated solid catalyst material. Gases required for the reaction are introduced into the top section of the reactor, and establishes a downward flow of reaction gas through the interior of the reactor column. Heat evolved by the reaction is removed from the column by vaporization of water surrounding the column, which is at saturation temperature. The reaction column consists of several sections of varying column diameter. In particular, the diameter of the upper part of the reaction column, where a relatively large amount of reaction heat is produced, is comparatively small; while the diameter of the lower portion of the column, where less reaction heat is liberated, is larger.
There remains, therefore, a current need for improved flow reactor apparatus for using a fixed heterogeneous catalyst which is effective in reducing the magnitude of the exotherm, reducing thermal degradation of catalyst activity and/or mechanical failure of catalyst/support, and thereby avoiding interruptions in service.
Advantageously, such improved flow reactor would, by means of higher selectivity and/or conversion of organic compounds, assist in improving recovery of value added products.